The relay nozzles in conventional air weaving looms generate a travelling field for transporting the weft thread. As the weft thread is exposed to the travelling field while moving through the weft thread insertion channel, a thread tension force is applied to the weft thread. The tension force depends on the flow speed of the air coming out of the relay nozzles. The tension force makes sure that the weft thread is stretched properly prior to beat up.
German Patent Publication 2,328,135 (Scheffel), published on Dec. 19, 1974, discloses a method for the weft thread insertion in nozzle weaving looms. The transport air is ejected by several relay nozzles distributed across the weaving width. The air or fluid streams are so controlled that the leading end of the particular weft thread is seized for transporting the weft thread, thereby pulling the weft thread through the insertion channel. More specifically, the relay nozzles are so controlled that the relay nozzle is switched on at the location where the leading end or tip of the weft thread travels at any particular instant. Thus, the nozzles according to the just mentioned German Patent Publication are referred to as pulling nozzles, although they are discharging pressurized air. As soon as the leading tip of the weft has passed the particular nozzle its air supply is switched off, with the exception of one or several nozzles distributed across the weaving width which continue to be supplied with pressurized air even after the weft thread tip has passed these nozzles. These few nozzles continue to receive pressurized air until the weft thread tip has passed entirely through the weaving width or rather through the insertion channel. This type of fluid flow control has shown in its practical application that tension force peaks occur at the end phase of the weft insertion, whereby weft breaks tend to occur. The cause for such weft breaks are believed to be due to the fact that the weft insertion speed is still relatively high even in the end phase of the weft insertion and that at the end of the weft insertion this relatively high speed must be abruptly reduced to zero by the action of the thread stopper which limits the weft thread length of each inserted weft thread.
When the weft thread is not inserted with the required speed, that is, if the speed of the flow medium that generates the weft pulling force is too low and the number of relay nozzles is too small, the result is a weft thread that is insufficiently tensioned which in turn causes the formation of loops which impair the fabric quality.
U.S. Pat. No. 4,759,392 (van Bogaert et al.), published on Jul. 26 , 1988, discloses a method and apparatus for controlling the operation of the relay nozzles on an air nozzle loom. The aim of the is known method and apparatus is to insert the weft thread with an optimal utilization of the airstream while using a minimal air volume to assure an insertion as perfect as possible. For this purpose, van Bogaert et al. disclose that the air nozzles are divided into groups. Initially, the nozzles of a first group are so controlled that a basic or first airstream for the weft insertion is generated, whereupon the nozzles of a second group of relay nozzles are controlled in such a manner that an additional or second airstream is generated which produces the tension force on the weft thread. The airstream of the nozzles in the first group has a speed which corresponds substantially to the programmed weft thread insertion speed . The airstream of nozzles forming the second group, however, has a substantially higher air speed than the given insertion speed for the weft thread.
Both methods and devices of the prior art discussed above disregard the fact that the product of the air flow speed V.sub.L effective on the weft thread , and the number n of the relay nozzles have a substantial influence on the size of the pulling force F.sub.G applied to the weft thread in addition to the influence of the speed V.sub.G of the weft thread itself and of the mass m of the weft thread. This means that depending on the size of the product V.sub.L .times.n in the relationship F.sub.G =f (V.sub.G, m, V.sub.L .times.n), weft thread breaks during the weft thread insertion will occur more frequently the larger this proportion V.sub.L .times.n is in the just mentioned relationship. In the foregoing relationship "f" means function of the elements recited in the parenthesis .
European Patent Publication 0,112,431 (Lerch), published on Jul. 4, 1984, discloses a method for the operation of an air nozzle loom in which at least one relay nozzle is reactivated prior to the end of a weft insertion but after it has been switched off following the passage of the weft thread tip. The reactivation takes place at least once for supporting and tensioning the weft thread. Where several relay nozzles are reactivated, they form a trailing or follow-up field of blowing flows. In both instances the duration of each blow of the initial travelling field of insertion blows along the entire insertion channel is shortened in its duration and the flowing force needed to carry the weft thread through the insertion channel is made up by the above mentioned reactivation of at least one reactivation nozzle. The total "on-time" of the nozzles is thus reduced for reducing the required air volume. However, the problem of weft breakage is not solved by the teaching of EPO 0,112,431.
Thus, there is room for improvement to avoid or at least substantially reduce weft thread breaks while still efficiently using the insertion fluid.